Type 9 is a low powered spark transmitter/receivr which was fitted into Cruisers and also in some Battlecruisers as the Auxiliary set for short distance communications when in company.Approximate distances achieved up to 25 miles. As the text in the Pre ""1 files show these sets (Type 3 in a Battleship) were the original "short distance set" and were used as we use VHF/UHF between ships operating in close proximity of one another. Clearly they were using the ground wave of a short wave transmission oblivious of where or what the associated skywave was doing! Read the PDF file under Further Reading which is a basic description of the Type 9. Many of the expressions used in the text of the PDF file have been covered and explained in the Pre WW1 files. DP means -difference in potential- (we say PD) - potential difference - today. Note under 'interference' the problems they had between the main wireless office (medium to high power transmissions) and the auxiliary wireless office (low power transmissions) especially when each separate aerial was quite large much more so than the aerials of today.